Case study:- Blow Moulding
Description

A middle-sized blow moulding company in the Netherlands, about 20 machines with screw diameters of 60-80 mm experienced very long flushing periods going from HDPE with carbon-black to any other colour. Purging of up to 16 hours with HDPE to get the last traces of carbon-black out was no exception.

Problem analysis

Blow moulding machines are infamous for their elaborate "downstream-plumbing", which contains many pressure-less zones. These zones are prone to have carbonised layers: because there is little movement of material in these areas, polymers burn quicker, and settle on the metal surface. Soon these layers carbonise, and form a rougher, more intrinsic, surface in which additives such as pigments or stabilisers get stuck easily. This becomes more noticeable when flushing with straight HDPE: lines of a pigment are formed in the parison.
Sometimes small parts of these layers break off, and you will notice these as black specks in your product.

Target

The target is not to eliminate the lines as quickly as possible, but twofold: to eliminate the source of the problem, and to keep this under control in the future. Due to their nature polymers burn, but the target is to keep this under control and to prevent carbonisation to build up. In other words: gels should be removed in the initial stage, when they can be removed easily and with little material.

Solution

The normal processing temperatures are 180 deg C, and as there is a carrier resin (in this case HDPE) available with a slightly lower MFI (0.2) than the processed material, we picked ECONOCLEAN 100-c. The die temperature was raised to 210 deg C, rpm's reduced to 20. The concentrate was processed according to instructions. Within 3 hours the extrudate had turned from completely black to white foamy. The test was carried out with Econoclean 100-c, as this company had enough carrier resin available. In all mentioned test the optimal result was achieved regarding cleaning, but more important, also in minimising costs through preventive maintenance.


Costcalculation

1.  A. current cleaning material:HDPE
     B. Used quantity (kgs)400 kg
     C. Price/kg:1,00 Euro
                Total material costs:400,00 Euro
     D. Cleaning time (hours):16
     E. Costs/hour:115,00 Euro
                Total costs machine time:1.840,00 Euro

      TOTAL COSTS CLEANING:
2.240,00 Euro

2.       A. ECONOCLEAN blend 
     B. Used quantity (kgs)30
     C. price/kg of blend:153 Euro
                Total material costs:153,00 Euro
     D. Cleaning time :3 hours
     E. Costs/hour:115,00 Euro
                Total costs machine time:345,00 Euro
      TOTAL COSTS CLEANING:498,00 Euro

      Difference between 1 and 2:       1.742,00 Euro = 77,8% cost reduction !

Note:

By applying preventive maintenance this way, not only are costs reduced, but also production hours gained, flexibility increased, and pulling the screw has been reduced to an absolute minimum!